The present invention relates to the diagnostic imaging arts. It particularly relates to computed tomography imaging employing a two-dimensional detector array that enables rapid acquisition of volumetric imaging data, and will be described with particular reference thereto. However, the invention will also find application in other types of radiation detectors for a variety of imaging applications employing x-rays, visible light, or other types of radiation. The invention will further find application in non-imaging radiation detectors for radiological applications and the like.
Computed tomography (CT) imaging typically employs an x-ray source that generates a fan-beam or cone-beam of x-rays that traverse an examination region. A subject arranged in the examination region interacts with and absorbs a portion of the traversing x-rays. A CT data measurement system (DMS) including a two-dimensional detector array in a cast frame assembly is arranged opposite the x-ray source to detect and measure intensities of the transmitted x-rays. Typically, the x-ray source and the DMS are mounted at opposite sides of a rotating gantry such that the gantry is rotated to obtain an angular range of projection views of the subject.
In helical CT imaging, the patient is advanced linearly through the examination region along a direction that is perpendicular to the gantry rotation plane to effectuate a helical orbiting of the x-ray source about the subject. X-ray absorption data obtained during the helical orbiting is reconstructed using filtered backprojection or another reconstruction method to generate a three-dimensional image representation of the subject or of a selected portion thereof.
The two-dimensional detector array of the DMS typically includes a scintillator crystal or array of scintillators which produce bursts of light, called scintillation events, responsive to impingement of x-rays onto the scintillator. A two-dimensional array of photodetectors such as photodiodes or photomultiplier tubes are arranged to view the scintillator and produce analog electrical signals corresponding to the scintillation events. Preferably, an anti-scattering module is precisely aligned and mounted in front of the scintillator to block scattered x-rays which contribute to measurement noise.
The analog electrical signals are routed via electrical cabling to a remote analog-to-digital converter which digitizes the analog signals. The digitized signals are multiplexed into a reduced number of transmission channels, and the transmission channels communicate the multiplexed digitized signals across the rotating gantry interface by a slipring arrangement.
DMS modules for CT imaging in the past have had a number of deficiencies relating to bulkiness and excessive mass, complex and non-standardized electrical wiring, parasitic noise coupling, complex and difficult optical alignment, and overall system complexity.
The anti-scatter module and the photodetector array must be closely aligned relative to one another and relative to the CT gantry. To achieve the required tolerances, in a conventional DMS the anti-scatter module, the scintillator, and the photodiode array are mechanically isolated from other components of the DMS and mechanically interconnected as a non-detachable captive assembly. Electrical coupling to the photodetector array is obtained by flexible electrical cabling that mechanically decouples the precisely aligned optics from other DMS components.
The amount of electrical cabling involved is substantial. An exemplary DMS having thirty-two rows of detectors (corresponding to thirty-two slices) and 672 detectors per row includes 21,504 detectors, each of which has its own electrical wiring which is brought together at an electronic signal processing module located remotely from the photodetector arrays.
Furthermore, there are many constraints on the arrangement of the electrical cabling. In addition to spatial limitations on and near the rotating gantry, the electrical cabling is further constrained by electrical path length requirements. Differences in the signal path lengths for transmitting the various detector outputs give rise to signal phase differences, data errors, different amounts of transmission noise, and signal delays due to differential signal transit times along paths of different lengths. The analog signals are also susceptible to parasitic noise pick-up if the cabling runs close to electrically active components including the DMS electronics and the x-ray source.
These factors typically lead to electrical routing which is unique for a particular CT scanner. To reduce bulkiness of the DMS due to the cabling, the electrical routing is typically taken off only one side of the DMS. This asymmetrical arrangement leads to signal path length differences. The asymmetrical arrangement can also lead to asymmetries in detector cooling which is corrected using dedicated heaters and fans to distribute heat over the photodetector arrays.
The lack of modularity of conventional DMS modules can also lead to unnecessary component replacement. For example, the captive anti-scatter module and scintillator are typically replaced along with the associated detector array, even though only the detector array may be malfunctioning.
The conventional DMS is typically heavy and bulky, as it includes a cast aluminum frame which supports separate substrates for the photodetectors (usually ceramic) and the remote electronic signal processing circuitry (usually arranged on printed circuit boards), massive quantities of cabling between the detector arrays and the signal processing circuitry, dedicated heaters and fans for temperature control, and electromagnetic shielding to reduce parasitic noise coupling onto the extensive electrical cabling.
The DMS weight and piecemeal construction also places limits on the attainable rotational speed of the gantry and on the data throughput, which in turn limit overall scan speed and the number of views per revolution. The complexity of conventional DMS modules impacts reliability and frequently necessitates field servicing by trained personnel for routine maintenance operations such as replacing and realigning detector array elements.
The present invention contemplates an improved apparatus and method that overcomes the aforementioned limitations and others.